Synthesis, structures, and magnetic properties of a series of new heterometallic hexanuclear Co₂Ln₄ (Ln = Eu, Gd, Tb and Dy) clusters

A series of new Co–Ln heterometallic clusters formulated as [Co₂Ln₄(μ₃-OH)₂(piv)₄(hmmp)₄(ae)₂]·(NO₃)₂·2H₂O (Ln = Eu (1), Gd (2), Tb (3), Dy (4), H₂hmmp = 2-[(2-hydroxyethylimino)methyl]-6-methoxyphenol, Hae = 2-aminoethanol, Hpiv = pivalic acid) were synthesized and characterized. X-ray crystallography reveals that each of them contains a heterometallic {Ln₄Co₂} core, which is supported by two μ₃-hydroxide, four piv⁻, four hmmp²⁻ and two ae⁻ ligands. The magnetic investigation indicates that 2 exhibits weak antiferromagnetic interactions between Gd^III ions, and a large MCE value of 24.9 J kg⁻¹ K⁻¹, while 4 shows a fast relaxation of magnetization. The TGA and VT-PXRD measurements suggest that all the compounds show good thermal stability and can be stable up to about 220 °C. In addition, to address the influence of the Gd–O–Gd angles on the magnetic properties, compound 2 was compared with a series of compounds involving different bridges between Gd^III ions. The comparison reveals that the tiny difference in the Gd–O–Gd angles favors different magnetic coupling.